Silicon nitride based ceramic cutting tools have become increasingly important in recent years for machining superalloys, steel and cast iron. Particular attention has been directed to ceramic composites containing silicon nitride, alumina and aluminium nitride, the so-called "SIALON" composites. These compositions are based on the solid solubility of both aluminum oxide and aluminum nitride in silicon nitride.
For example, Jack, et al., U.S. Pat. No. 3,991,116 discloses a ceramic material containing at least 90% of SIALON. Subsequent work has focused on combining SIALON materials with various additives to improve cutting performance. For example, Lumby, et al., U.S. Pat. No. 4,323,323 discloses a single phase SIALON compound and a second phase containing a rare earth metal such as yttria. Komatsu, et al., U.S. Pat. No. 4,327,187 discloses a sintered ceramic body containing SIALON and an oxide of titanium, magnesium or zirconium.
Efforts have been made to improve the cutting characteristics of SIALON based ceramics by also employing an adherent coating on the ceramic composite. For example, Sarin, et al., U.S. Pat. No. 4,424,066 discloses the use of an adherent alumina coating layer on a ceramic composite substrate body and Sarin, et al., U.S. Pat. No. 4,426,209 discloses an adherent refractory metal carbide layer.
In addition to the use of additives and coating layers, efforts have been made to improve the processing of the SIALON containing composite. Lumby, et al., Reexamined U.S. Pat. No. B1 4,127,416 discloses a pressureless sintering process for forming a ceramic product containing at least 80% of a single phase SIALON compound. Another method of forming a SIALON ceramic material is disclosed in Lumby, et al., U.S. Pat. No. 4,113,503 which requires the formation of a ceramic intermediary which is then ground into a powder and heated with or without pressure in the presence of silicon nitride and silica.
Buljan, et al., U.S. Pat. No. 4,421,528 describes composite cutting tools produced by sintering a mixture of specified amounts of silicon nitride, aluminum oxide and yttria and then combining the sintered product with a hard refractory material such as titanium nitride. Despite these efforts, there is still room for significant improvement in the cutting characteristics of SIALON based composite cutting tools.
It is therefore an object of the invention to provide a SIALON based ceramic composite which is particularly suited for the machining of superalloys, steel and cast iron.
It is another object of the invention to provide a ceramic composite containing SIALON which is less reactive with the workpiece than known SIALON composites.
It is a further object of the invention to provide a SIALON ceramic composite containing a uniform dispersion of titanium nitride to improve cutting efficiency.